In the conventional shell and tube type heater exchanger for transferring heat from one fluid to another, there is generally provided a cylindrical shell enclosing a plurality of parallel tubes supported at their ends by tube sheets. The tube sheet is usually a perforated circular metallic plate to which the ends of the tubes are metallurgically bonded as by welding.
Since one fluid is usually at a higher temperature than the other fluid at almost all locations of the walls that separate them, mechanical stresses are caused by thermal expansion when the temperature of one or both fluids change as may be occasioned by the actual temperature of the fluid or the rate of flow. The stresses created are high in situations where the metallic portions of the heat exchanger are restrained from expanding freely. Thermal stresses are generally comparatively high in thick metals that undergo sudden change in temperature of one face because the effect of expansive force is immediate whereas considerable time is required for conduction of heat through thick portions, and thick portions are self-retraining by reason of their thickness. Thus, it is possible for the tube sheet due to its lower mass to change its temperature in 10 seconds whereas adjacent portions of the shell might not reach the new temperature for 5 to 10 minutes.
Tube sheets are usually quite thick by comparison with the tubes or with the shell. Internal thermal stresses are often quite high in tube sheets. Severe changes in flow conditions or temperature may cause stresses to be severe in the tube sheet. When a heat exchanger goes through many cycles of temperature change, the reversal of thermal stresses may eventually cause the metal to crack and leak at the boundry between the shell and the tube sheet.